1. Field of the Invention
The present invention relates to a motor driving apparatus having a fault diagnostic function.
2. Description of the Related Art
Providing a motor driving apparatus, which is connected to a three-phase AC power supply and which comprises an AC/DC converter for converting three-phase AC power supplied from the three-phase AC power supply into DC power and a DC/AC converter for converting the DC power output from the AC/DC converter into three-phase AC power for supply to a motor, is known. In such a motor driving apparatus, if three current sensors are provided that detect the three-phase output currents independently of each other, the apparatus can reliably detect any overcurrent occurring in the DC/AC converter and can also identify whether the cause is a ground fault or a phase-to-phase short circuit. That is, if overcurrent occurs in the DC/AC converter because of a ground fault or a phase-to-phase short circuit, the overcurrent can be detected by one of the three sensors, and if the sum of the currents detected by the three current sensors is not zero, it is determined that the cause is a ground fault, but if the sum is zero, it is determined that the cause is a phase-to-phase short circuit.
However, generally only two sensors are provided in order to reduce the cost, and the remaining one of the three phase currents is obtained through calculation by assuming that the sum of the three phase currents is zero. As a result, in the case of a conventional DC/AC converter having two output current sensors, there are cases where overcurrent caused by a ground fault cannot be detected, as will be described in detail later, and if the overcurrent can be detected, it is not possible to identify whether the cause is a ground fault or a phase-to-phase short circuit.
Accordingly, in the case of such a DC/AC converter having two output current sensors, the operation of the apparatus has to be stopped in the event of the occurrence of an overcurrent alarm, and the insulation resistance of the motor and power lines has to be checked in order to determine whether the cause is a ground fault or a phase-to-phase short circuit. Further, if the ground fault is intermittent, it is difficult to identify the cause.
On the other hand, in the case of a motor driving apparatus provided with three output sensors by adding an extra current sensor as earlier described, if the apparatus is one constructed to drive a plurality of motors by supplying the DC power output from the AC/DC converter to a plurality of DC/AC converters, the extra current sensor has to be added to each of the plurality of DC/AC converters, which greatly increases the cost.
JP7-239359A discloses an apparatus that uses one or two current sensors and that provides protection against ground faults by detecting the presence or absences of a ground fault before starting the operation by applying a ground-fault detecting control signal to the AC/DC converter. However, no description is given of how a distinction is made between a ground fault and a phase-to-phase short circuit.
On the other hand, JP4-10536A discloses an apparatus in which a zero phase current detector is provided between the three-phase AC power supply and the apparatus, with provisions made to stop the operation of the inverter apparatus upon detection of a ground fault. However, no description is given of the detection of a phase-to-phase short circuit.
JP63-85380A cited in JP7-239359A discloses a method that provides a detector for detecting the DC current input to the inverter apparatus, wherein the current detection pattern generated when a test control signal was applied to the inverter apparatus before starting the operation is compared with a known pattern to detect the presence or absence of a ground fault or a short circuit fault. However, this method not only has the drawback that the detected current contains a reactive current that flows through free wheel diodes in the inverter apparatus, as described in JP7-239359A (paragraph 0004), but also has the problem that a ground fault or a phase-to-phase short circuit that occurs after starting the operation or a ground fault that occurs intermittently during the operation cannot be detected.